Secure covert combat identification friend-or-foe (IFF) system for the dismounted soldier

ABSTRACT

A combat IFF system, for use in a combat exercise or on the battlefield, including a helmet-mounted passive IFF response unit and a weapon-mounted IFF interrogatory unit for each soldier. Infrared (IR) signals are employed for both challenge and response. The IR response signal is a very narrowly-targeted reflection of the relatively narrow IR transmit signal, thereby minimizing interception opportunities. The transmit and response signals are encoded in a transaction that cannot be compromised even when either or both signals are intercepted and decoded by the enemy. The combat IFF system includes biometric anti-spoofing features that prevent any use by an enemy in possession of captured units. Military radio-frequency (RF) spectrum is not required so there are no bandwidth limitations on simultaneous IFF transactions in the battlefield. A combat IFF transaction is completed in milliseconds.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to combat identification systems forthe dismounted soldier and more particularly to a secure covertidentification as friend or foe (IFF) system for interrogating adismounted soldier with a coded infrared (IR) signal that is selectivelyretroreflected and encoded by the target when recognized as a validchallenge.

2. Description of the Related Art

Dismounted Armed Forces have an interest in the remote and secureidentification of a person as friend or foe, during combat trainingexercises and in armed conflicts. Identification as friend or foe (IFF)systems are well-known in the art for military aircraft and otherweapons systems. Such systems are useful for preventing action againstfriendly forces. The military platform commanders on a modernbattlefield must accurately identify potential targets as friend-or-foe(IFF) when detected within range of available weapon systems. Suchtarget IFF presents a difficult decision for a military platformcommander, who must decide whether to engage a detected target whileavoiding accidental fratricide. This problem is even more difficult forthe dismounted soldier who may be moving covertly through an unknowncombat zone at night with limited visibility. Simple visual assessmentsof other dismounted soldiers is not a reliable IFF method for militaryplatforms or dismounted infantry.

The art is replete with proposals for IFF systems for military platformsin modern land battlefields. But commanders often still rely onlow-resolution visual and infrared images to identify detected targets.Commanders often must operate under radio silence to avoid detection byan enemy. With infrared (IR) imagers alone, the identification ofindividual dismounted soldiers is not feasible, although the IRsignatures of land vehicles may have some use. IFF systems that requireone or more radio signals are limited in channel-capacity and must bearthe overhead of selecting and/or awaiting an available battlefieldchannel before completing the IFF task. Active-response systems requirethe emission of a signal by the unknown respondent in response to averified challenge, which may compromise the security of bothinterrogator and respondent. Active transponders are subject to captureand may be used for spoofing by the enemy in a battlefield or a combattraining environment. Passive response systems rely on the return of anecho (reflection) of a challenge signal to the interrogator, but simplereflection schemes are easily compromised and more elaborate passivereflection schemes are still subject to intercept, compromise or capturefor use by the enemy in spoofing the interrogator.

As described in U.S. Pat. No. 4,851,849 by Otto Albersdoerfer, a typicalactive IFF technique for vehicles is to equip a military vehicle with atransponder that emits a coded return signal when an interrogating radarpulse is detected by its receiver. As described in U.S. Pat. No.5,686,722 by Dobois et al., a more sophisticated active IFF techniquefor vehicles uses a selective wavelength optical coding system withtunable optical beacons mounted on each vehicle. By spreading theoptical broadcast energy into frequency in a precise manner, the beaconidentifies the host vehicle to friendly receivers while remaining covertto the enemy.

As described in U.S. Pat. No. 4,694,297 by Alan Sewards, a typicalpassive IFF technique for vehicles is to equip a military vehicle with apassive antenna that reflects an interrogatory radar beam while adding adistinctive modulation by varying the antenna termination impedanceresponsive to evaluation of the interrogatory beam. A more sophisticatedpassive electro-optical IFF system for vehicles is described in U.S.Pat. No. 5,274,379 by R. Carbonneau et al. wherein each friendly vehicleis provided with a narrow-beam laser transmitter and a panoramicdetector. If a vehicle detects a coded interrogator laser beam andidentifies the code as friendly, it opens a blocked rotatingretro-reflector to clear a reflection path back to the source, where itcan be identified by another narrow field-of-view detector. A furthermodulation is also added to the reflected beam to identify thereflecting vehicle as friendly. If an improperly coded beam is detected,the transmission path is not cleared, thereby preventing reflection ofthat beam and warning is sent to the vehicle commander of an unfriendlylaser transmission. Others have proposed similar passive optical IFFsystems for vehicles, including Wooton et al. in U.S. Pat. No. 5,459,470and Sun et al. in U.S. Pat. No. 5,819,164.

The art is less populated with IFF proposals for the lone dismountedsoldier (the infantryman on foot). Whether in actual combat or in atraining exercise, the dismounted soldier operates with severe weightlimits and little onboard electrical power. The friendly foot soldierhas no distinctive acoustic, thermal or radar cross-section that may beused to assist in distinguishing friendlies from enemies. But somepractitioners have proposing IFF solutions for the dismounted soldier,both active and passive. For example, in U.S. Pat. No. 6,097,330, Kiserproposes an active IFF system for identifying concentrations of groundtroops (or individuals) from an aircraft by interrogating a (heavy)human-mounted radio transmitter carried by one of the group with anarrow-cast optical signal. As another example, in U.S. Pat. No.5,299,277, Rose proposes a compact active IFF system to be carried byeach individual dismounted soldier for use in combat exercises or on thebattlefield. The system includes a clip-on beacon and a hand-held(flashlight-style) or weapon-mounted detector. The beacon radiates aspread-spectrum low-probability-of-intercept (LPI) signal at opticalfrequencies that are selected to be invisible to the usual detectorspresent in the battlefield. Rose doesn't consider the problem ofspoofing with captured devices. As yet another example, in U.S. Pat. No.5,648,862, Owen proposes an active IFF system implemented by addingprovisions for coded two-way transmissions to the night-vision systemsoften worn by dismounted soldiers. As a final example, in U.S. Pat. No.5,966,226, Gerber proposes an active combat IFF system for eachdismounted soldier that includes a weapon-mounted laser projector forinterrogating suspected targets and a harness including means forreceiving the interrogatory signal and means for responding with anencoded radio, acoustic or optical signal. But these proposals do notresolve the spoofing problem (through capture of a beacon or harness,for example) and are not particularly covert because the respondingtarget generally broadcasts an active signal either continuously or inresponse to interrogation. Any IFF proposal employing broadcast signalsalso faces a battlefield channel capacity (or channel availabilitydelay) problem as well.

There is still a clearly-felt need in the art for an IFF system for thedismounted soldier that provides true passive covertness and that cannotbe spoofed under any battlefield conditions. The desired IFF systemrequires little power and is adapted to prevent any use of capturedequipment or intercepted signal codes. Finally, the system should beinexpensive enough to permit equipping every soldier with the necessaryinterrogation and response equipment for combat exercises or actualbattlefield conditions. These unresolved problems and deficiencies areclearly felt in the art and are solved by this invention in the mannerdescribed below.

SUMMARY OF THE INVENTION

This invention solves the above-described problems by providing asystem, for use in either a combat exercise or on the battlefield, thatincludes a passive helmet-mounted identification as friend or foe (IFF)response unit and a weapon-mounted IFF interrogatory unit for eachsoldier. Infrared (IR) signals are employed for both challenge andresponse. The IR response signal is a very narrowly-targeted reflectionof the relatively narrow IR transmit signal, thereby minimizinginterception opportunities. The transmit and response signals areencoded in a transaction that cannot be compromised even when either orboth signals are intercepted and decoded by the enemy. The combat IFFsystem includes biometric security (anti-spoofing) features that preventany use by an enemy soldier in possession of captured units. No militaryradio-frequency (RF) spectrum is used so there are no limitations onsimultaneous IFF transactions in the battlefield. A combat IFFtransaction is very brief and may be completed within the typical humanreaction time interval.

It is a purpose of this invention to provide a secure covert combat IFFsystem adapted for use either in a combat exercise or on thebattlefield. It is an advantage of this invention that the IFFtransaction employs line-of-sight IR signals that are not likely to besurreptitiously intercepted. It is a feature of this invention that bothtransmit and response signals are encoded for security in a manner thatprevents compromise even if one or both are intercepted and decoded byan enemy. It is another advantage of this invention that the responseunit is entirely passive and cannot radiate any energy that may beintercepted.

It is another purpose of this invention to provide a combat IFF systemhaving a reduced cost and weight without using any additional militaryRF spectrum. It is a feature of this invention that the response unit isintegrated into the helmet of the dismounted soldier, adding littleweight thereto. It is an advantage of this invention that theinterrogatory unit and the response unit generally include commercialoff-the-shelf (COTS) components available in quantity at relatively lowcost and the response unit may be powered by a small lithium-ion batteryfor up to three years. It is another feature of this invention that theentire IFF transaction occurs in the optical spectrum, using no militaryRF spectrum.

It is yet another purpose of this invention to provide a combat IFFsystem that cannot be compromised by the enemy, whether by interception,spoofing or capture. It is a feature of this invention that thehelmet-mounted response unit is automatically disabled upon doffing ofthe helmet and may be reactivated only by means of selected biometricdata unique to the soldier fitted with the helmet; and similarprovisions may be made for the interrogatory unit. It is an advantage ofthis invention that an IFF transaction cannot be compromised or spoofedeven by an enemy in possession of both decoded signals together withcaptured interrogatory and response units.

In one aspect, the invention is a method for identifying as friend orfoe a combat response unit having a helmet-mounted challenge receiverand a retroreflector obturator, including the steps of projecting an IRtransmit signal including a transmitted code of the day (TCOD) onto thecombat response unit from a combat interrogatory unit, receiving the IRtransmit signal and TCOD at the challenge receiver, selectivelyreflecting the IR transmit signal by opening and closing theretroreflector obturator according to a response code of the day (RCOD),receiving the reflected IR transmit signal and RCOD at the combatinterrogatory unit, and combining the received RCOD with the TCOD toidentify the combat response unit as friend or foe.

In a preferred embodiment, the invention is a system for combat IFFcommunications including a combat interrogatory unit having projectormeans for projecting an IR transmit signal including a TCOD, receivermeans for receiving a reflected IR transmit signal including a RCOD, andmeans for combining the received RCOD with the TCOD to identify thesource of the reflected IR transmit signal as friend or foe; and ahelmet-mounted combat response unit having sensor means for receiving aprojected IR transmit signal including the TCOD, retroreflector meansfor reflecting an incoming IR transmit signal generally back along theincoming path thereof, obturator means for obstructing theretroreflector means to prevent reflection thereby, and means foropening and closing the obturator means according to the RCOD.

The foregoing, together with other objects, features and advantages ofthis invention, can be better appreciated with reference to thefollowing specification, claims and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is nowmade to the following detailed description of the embodiments asillustrated in the accompanying drawing, in which like referencedesignations represent like features throughout the several views andwherein:

FIG. 1 is a sketch illustrating the operation of the combatidentification as friend or foe (IFF) system of this invention;

FIG. 2 is a sketch of the helmet-mounted combat response unit of thisinvention illustrating the IR sensor and retroreflector/obturator array;

FIG. 3 is a sketch of the combat interrogatory unit of this inventionillustrating a preferred weapon-mounted disposition thereof;

FIG. 4 is a schematic diagram illustrating the optical operation of thecombat response unit of FIG. 2;

FIG. 5 is a schematic block diagram illustrating the detailed operationof the combat response unit of FIG. 2;

FIG. 6 is a schematic block diagram illustrating the detailed operationof the combat interrogatory unit of FIG. 3;

FIG. 7 is a chart illustrating the relationship between examples of thecode of the day (COD), the transmitted code of the day (TCOD) and theresponse code of the day (RCOD) according to the method of thisinvention;

FIGS. 8( a)-(d) are sketches illustrating several examples of the meansfor accepting biometric data employed in the combat response unit ofFIG. 2; and

FIG. 9 is a block diagram of a flow chart illustrating the IFF method ofthis invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates the operation of the combat identification as friendor foe (IFF) system of this invention. The combat interrogatory unit 20of this invention is shown in a preferred weapon-mounted dispositionwhere the challenging soldier may target the helmet-mounted combatresponse unit 22 of this invention worn by a soldier in a combatsimulation exercise or in actual combat. An infrared (IR) transmitsignal 24 is projected by unit 20 upon operator command. Transmit signal20 radiates outward along a narrow beam, eventually illuminatingresponse unit 22. For example, transmit signal 24 may be embodied as a 6milliradian beam of 1540 nm IR light, which then illuminates an area ofabout 6 meters on a side at a typical weapon range limit of 1000 meters.Upon being received, detected and verified at response unit 22, transmitsignal 24 is then retroreflected back to interrogatory unit 20 as theresponse signal 26. For a 6 milliradian transmit signal 24, responsesignal 26 includes a reflection of, for example, a 1.5 centimeterportion of the 6 meter transmit beam 24. This 1.5 cm reflected portionincludes about 0.002 percent (−47 dB) of the initial energy of transmitsignal 24, which is generally reflected back to interrogatory unit 20 bya precision retroreflector (see FIGS. 2 and 5). Response signal 26 isreceived at interrogatory unit 20 reduced by an additional −8 dB, whichleaves sufficient power for IFF detection and processing atinterrogatory unit 20. The 1540 nm signal wavelength is preferredbecause it is eye-safe and has relatively low absorption and scatteringloss in the usual battlefield smoke and haze.

FIG. 2 illustrates helmet-mounted response unit 22 in some additionaldetail, illustrating the IR sensor and retroreflector array 28consisting of an IR sensor 30 and a retroreflector/obturator 32 inalternating disposition. Retroreflector/obturator 32 provides theprecision return along the incoming path of any ray arriving from adirection within a 90 degree cone of coverage. Array 28 may includetwelve sensors 30 and retroreflector/obturators 32, alternated todispose one of each for every 50-degrees of azimuth, thereby providingthe desired 360-degree hemispherical coverage from 90-degrees above thehorizon to 30-degrees below the horizon. Array 28 is fixed to theresponse unit 22 by any useful means, such as, for example, rivets,cement or stitching (not shown), and may be oriented asymmetrically withrespect to the vertical axis to enhance the desired hemisphericalcoverage above the horizon and down to 30 degrees below the horizon. Theentire helmet-mounted response unit 22 weighs about 230 grams andrequires no power except for the low-power electronic logic componentsdescribed below in connection with FIGS. 4-5, which may be powered forup to 3 years by a small lithium-ion battery. All elements described inconnection with the exemplary embodiment of response unit 22 areavailable as COTS products, including the necessary battery.

Retroreflector/obturator 32 may employ any useful retroreflector deviceknown in the art, such as, for example, one of the line of Tech Spec™Corner Cube Retroreflectors (Trihedral Prisms) available from EdmondIndustrial Optics, Barrington, N.J. The obturator portion ofretroreflector/obturator 32 may include a mechanical shutter devicecapable of cycling open and closed within a few milliseconds, or morepreferably, a liquid crystal device (LCD) disposed over theretroreflector portion, such as the LCD-CDS92106 available from CubicDefense Systems, San Diego, Calif.

FIG. 3 illustrates interrogatory unit 20 in a preferred weapon-mounteddisposition. Interrogatory unit 20 is removably fixed by a clamp 34 to aweapon barrel 36, which in use is removably fixed to a weapon carried bya dismounted soldier (FIG. 1), and includes an IR projector 38 forprojecting IR transmit signal 24 and an IR receiver 40 for receivingresponse signal 26, which is a reflected portion of IR transmit signal24. IR projector 38 may include, for example, a 1540 nm laser diodesuitable for sharing with a laser range finder system such as the750-Series laser rangefinder receivers available from Analog Modules,Inc., Longwood, Fla. IR projector 38 is preferably factory-aligned withIR receiver 40 for ease of boresighting and operation in the field.Interrogatory unit 20 may also include an IFF indicator 42 forindicating the results of an IFF challenge transaction, and a modeswitch 44, for changing selected operating parameters, such as the IFFencoding scheme, signal timing, power on/off or IR projector wavelength,for example. Interrogatory unit 20 also includes a remote interrogationswitch (not shown) that may be removably fixed to any convenient spotproximate the weapon trigger area (not shown) for convenient accessduring use for interrogation of unidentified response units. This remoteinterrogation switch may also include means for accepting biometric data(not shown, see FIG. 8) representing an operator fingerprint (or irispattern or the like) for security purposes such as, for example,disabling (by a time-out timer) a captured unit to prevent unauthorizeduse by an enemy soldier. Provision may be made for either local batteryor vehicle-cable power and/or for coupling to digital vehicle-systemsfor any useful purpose.

The two-way signal-to-noise budget for an IFF transaction may beappreciated with reference to the following example. A +44 dBm IRtransmit signal level may be provided at 1540 nm by a commercialoff-the-shelf(COTS) class-i laser (25 watts). At an exemplary 1000 meterrange limit, IR transmit signal 24 arrives at response unit 22 with apower of −8 dBm, having lost, for example, 52 dB en route throughspreading, absorption and scattering losses. IR transmit signal 24 isthen reflected by the appropriate retroreflector/obturator 32 andreturned to IR receiver 40 with a −24 dB spreading loss (less thansimple spherical spreading because of the retroreflector properties), a−3 dB obturator insertion loss, and scattering and absorption losses ofless than −1 dB, leaving about −36 dBm at IR receiver 40, which isperhaps 8-10 dB above the noise threshold of an exemplary COTS laserdetector, such as the Analog Modules No. 754 available from AnalogModules, Inc., Longwood, Fla. Even in a battlefield smoke and hazecondition, the 1540 nm laser signal suffers only an additional −7.4 dBof scattering loss over the 1000 meter two-way path, leaving perhaps 1-3dB of signal-to-noise ratio (SNR) margin in the IFF transaction of thisexample. The inventors have calculated link margins showing that IFFtransactions are 99.5% successful in clear air at 1540 nm with a 5 dBmargin (8 dB is needed for smoky haze).

FIG. 4 is a schematic diagram illustrating the optical operation ofresponse unit 22 (FIG. 2). As described in detail hereinbelow inconnection with FIG. 7, transmit signal 24 (FIG. 1) includes a transmitcode of the day (TCOD) including a frame-synchronization preamble (notshown) followed by a TCOD 24(a) followed by a TCOD interrogation pulsestream 24(b). In operation, TCOD 24(a) is received by one or more of theplurality of IR sensors 30 and presented to the challenge receiver 46for verification. When TCOD 24(a) is verified, challenge receiver 46produces a response code of the day (RCOD) signal 48, which includes alogical combination of selected information from TCOD 24(a) and from thelocal memory of challenge receiver 46 (not shown) that is described indetail hereinbelow. RCOD signal 48 is presented to the obturator driver50, which produces an obturator signal 52 for opening and closing(making translucent and opaque) the obturator 54 in each of theplurality of retroreflector/obturators 32. A biometric identification(ID) signal 56 is also presented to obturator driver 50 to enable ordisable operation thereof based on the verification of a scannedthumbprint input by the dismounted soldier in possession ofhelmet-mounted response unit 22, which is further described hereinbelowin connection with FIGS. 5 and 8( a)-(d). RCOD signal 48 includes adelay and a response pulse stream 26(a). The RCOD delay is sufficient topermit TCOD interrogation pulse stream 24(b) to arrive atreflector/obturator 32. Obturator 54 is then cycled open and closed inaccordance with obturator signal 52 to produce response pulse stream26(a) by reflecting selected elements of interrogation pulse stream24(b) from the retroreflector 58 in at least one of the plurality ofretroreflector/obturators 32.

FIG. 5 is a schematic diagram illustrating the logical operation ofresponse unit 22 (FIG. 2). The elements of FIG. 5 are arranged in threefunctional categories for clarity; (a) helmet sensors, (b) helmet logicand (c) helmet memory. In operation, helmet-mounted response unit 22 isfirst programmed with certain biometric data unique to the user and witha code of the day (COD), which includes a challenge code (CC) followedby a response code (RC). The helmet is then donned by the user accordingto a predetermined anti-spoofing protocol. The donning soldier firstpasses his/her thumb over a biometric thumbprint sensor 60 in a manner(known only to the soldier) for which response unit 22 has previouslybeen programmed. Biometric thumbprint sensor 60 produces the codedbiometric data stream 62, which is immediately evaluated by thebiometric identification logic 64. This evaluation involves comparingdata stream 62 to the earlier-stored thumbprint scan data 66 for thedonning soldier to produce an enable/disable decision. If logic 64decides that the thumbprint scan fails, the soldier may be afforded, forexample, two additional opportunities to enable the helmet-mountedresponse unit by means of an audible or visual indicator (not shown).After two additional attempts, logic 64 decides that the biometric ID isinvalid, and sets biometric ID signal 56 to “disable” obturator driver50, permanently shutting obturator 54 to prevent any retroreflection.Moreover, biometric ID signal 56 may also be presented to aself-destruct timer 67, which begins a logic and memory self-destructioncount-down. When completed, this countdown triggers the erasure of alllogic and data stored in the helmet memory, thereby making a capturedhelmet-mounted response unit worthless to an enemy. The helmetelectronics may easily be reprogrammed by the friendly forces for use inanother combat exercise or the like.

After successfully completing a thumb scan and donning the helmet,biometric ID signal 56 is set to “enable” and obturator driver 50 isenabled so the soldier is thereafter equipped to passively respond tovalid incoming interrogatory signals in the manner now described. Whenthe soldier again doffs the helmet, the helmet-doffing sensor 68immediately signals logic 64, which resets biometric ID signal 56 to“disable” and obturator driver 50 is again disabled, permanentlyshutting obturator 54 to prevent any retroreflection.

In use, response unit 22 first begins the IFF transaction upon thearrival of IR transmit signal 24 from interrogatory unit 20 (FIGS. 1 and6). TCOD 24(a) is received by at least one IR sensor 30, which producesa signal 70 that is presented to the signal verification and decodinglogic 72. The relative location on the helmet of the particular IRsensor 30 receiving the signal is identified in logic 72 to determinethe arrival quadrant (Front, Right, Rear, or Left) of IR transmit signal24. This arrival quadrant information may be used in logic 72, forexample, to bias other IF processing parameters or to signal thehelmeted soldier, by audible or visual means, of the arrival quadrant sothat appropriate action may be undertaken (such as, for example,skeptical evasion of IFF interrogatories arriving from behind enemylines). TCOD 24(a) is validated by examining it to verify that itincludes a valid CC followed by a randomly-generated generated number(RGN). This is accomplished by comparing the received CC with the CCportion of the COD 74 stored in the helmet and accepting the RGN if theincoming CC matches the stored CC. The entire stored COD 74 and theincoming RGN are then passed as a data signal 76 to the response code ofthe day (RCOD) generation logic 78, which creates the RCOD 80 bycombining the CC and RC portions of the stored COD with the PRN in apredetermined manner. After generation, RCOD 80 is passed to obturatordriver 50 for use in opening and shutting obturator 54 accordingly. Thefirst portion of RCOD 80 is a closed-shutter interval that is computedas a combination of the stored RC and the incoming RGN. The secondportion of RCOD 80 is an open-shutter interval that is determined bysome portion of the CC. RCOD 80 is synchronized to begin at apredetermined interval (such as 10 ms) after the end of the incomingTCOD so that the interrogator may evaluate RCOD 80 to verify that thereflecting response unit is a friend and not a foe, thereby completingthe IFF transaction.

FIG. 6 is a schematic diagram illustrating the logical operation ofinterrogatory unit 20 (FIG. 3). The elements of FIG. 6 are arranged inthree functional categories for clarity; (a) interrogatorsensors/emitters, (b) interrogator logic and (c) interrogator memory. Inuse, interrogatory unit 20 first begins the IFF transaction when thetrigger sensor 82 detects a command from the operator to interrogate atarget. Trigger sensor 82 produces a signal 84 that is presented to theTCOD generation logic 86, which responsively retrieves a locally-storedcopy of the COD 88 and a RGN 90 from a pseudorandom number (PRN)generator 92. TCOD generation logic 86 then combines the CC portion ofCOD 88 with RGN 90 to produce the TCOD portion of TCOD 94, which is thenfollowed by the TCOD interrogation pulse stream. TCOD 94 is thenamplified as necessary to drive the TCOD projector 96, which producesthe IR transmit signal 24, initiated with the frame-synchronizationpreamble (not shown), and directs it to the target. TCOD is alsopresented to the signal decoder and verifier logic 98 for use inprocessing any incoming IR signals received at the IR sensor 100. Whenthe TCOD portion of TCOD 94 is transmitted (see FIG. 7), logic 86notifies logic 98 of the start of the response interval. Any incomingresponse signal 26 at RCOD sensor 100 produces a RCOD signal 102 that ispresented to logic 98 for evaluation. Logic 98 reverses the encodingprocess discussed above in connection with FIG. 5 and below inconnection with FIG. 7. RGN 90 and the RC from COD 88 are used tocompute the proper closed-shutter interval, which is compared to thedelay interval between completion of the transmission of TCOD 94 and thebeginning of response pulse stream 26(a). The appropriate elements ofCOD 88 are then used to compute the proper open shutter interval, whichis then compared to the actual length of the incoming response pulsestream 26(a). If the two comparisons are not successful, logic 98 doesnothing; leaving the IFF display 104 inactive. When the comparisons areboth successful, logic 98 produces an IFF indicator signal 106, whichactivates IFF display 104, thereby completing the IFF transaction. IFFdisplay 104 may be a simple light-emitting diode (LED) indicator or anaudible signal or any other useful indicator consistent withrequirements for security and covertness.

As described above in connection with FIG. 5, interrogatory unit 20 mayalso include anti-spoofing means to prevent use by an enemy of acaptured interrogatory unit. A biometric sensor 108 may be used toaccept thumbprint or fingerprint input or retinal image input or anyother useful biometric data unique to the authorized operator. Uponactivation, biometric sensor 108 presents the incoming biometric data110 to the biometric ID logic 112 for comparison to the biometric IDdata 114 stored previously by the same soldier. If, at some point, logic112 decides that the biometric ID is invalid, a biometric ID signal 116is set to “disable” and TCOD generator logic 86 is disabled, permanentlyshutting down TCOD projector 96. Moreover, biometric ID signal 116 mayalso be presented to a self-destruct timer 118, which begins a logic andmemory self-destruction count-down. When completed, this countdowntriggers the erasure of all logic and data stored in the interrogatormemory, thereby making a captured weapon-mounted interrogatory unitworthless to an enemy. The interrogator electronics may easily bereprogrammed by the friendly forces for use in another combat exerciseor the like.

After successfully completing a biometric scan, biometric ID signal 116is set to “enable” and TCOD generator logic 86 is enabled so the soldieris thereafter equipped to actively interrogate targets in the mannerdescribed above. After a predetermined period of inactivity, establishedby, for example, a simple timer (not shown), logic 116 resets biometricID signal 116 to “disable” and TCOD generator logic 86 is againdisabled, permanently shutting down TCOD projector 96.

FIG. 7 is a chart illustrating the relationship between exemplaryembodiments of the COD 120, the TCOD 122 and the RCOD 124 of thisinvention. Each of the six symbols of COD 120 and TCOD 122 are 7-bitvalues (ranging from 0-127) encoded as pulse positions within a 36microsecond frame interval, which is established and synchronized foreach IFF transaction by a three-pulse TCOD frame-synchronizationpreamble (not shown). Each pulse is 66.67 ns in duration, which allowsthe use of an inexpensive 15-MHz microprocessor (not shown) forimplementing the various logic described herein. The position of a pulsewithin a frame represents the corresponding 7-bit symbol. COD 120 ischanged daily and all interrogatory units and response units in an IFFsystem must be updated daily with the new COD 120. COD 120 includes a4-symbol CC 126 and a 2-symbol RC 128. TCOD 122 includes CC 126 and a2-symbol RGN 130 followed by a 10-ms buffer interval followed by TCODinterrogation pulse stream 132 consisting of a 50 ms stream of 1-msspaced pulses. RCOD 124 includes a delay 134 during the closed-shutterinterval followed by a response pulse stream 136 during the open shutterinterval. Delay 134 is computed by dividing the absolute value of thedifference between RGN 130 and RC 128 by ten and adding 10 ms, providinga range of values from 10-20 ms in 1 ms steps. This coding method isboth secure and covert because RGN 130 is shared only by theinterrogator and target even though COD 120 is known by all friendlies.Although interception of RGN 130 is unlikely because of the narrow TCODbeam (3 milliradians), any such interception is useless without priorknowledge of COD 120 because RC 128 is not transmitted and both RC 128and RGN 130 are required to compute delay 134. The open shutter intervalthat reflects response pulse stream 136 is computed in milliseconds byadding three times the fourth digit of CC to five, giving a range ofvalues from 5 to 32 ms in 3 ms steps.

Thus, the duration of response pulse stream 136 cannot be spoofedwithout prior knowledge of COD 120 or interception of TCOD 122 and theduration of delay 134 cannot be spoofed without having both the priorknowledge of COD 120 and the successful interception of TCOD 122. Suchspoofing also presumes prior knowledge of the decoding algorithmsdescribed above. Each IFF transaction is completed within 60 ms. Aninterrogator may repeat an interrogation as desired to reduce theprobability of false negatives. Assuming that interrogation is repeatedup to four times before accepting a negative (partial response) result,the entire IFF transaction is completed within 250 ms. This method ofobtaining a response is necessary only under extreme scintillation andrange conditions, at the limits of link performance.

FIGS. 8( a)-(d) show several examples of suitable means for acceptingbiometric data for helmet-mounted response unit 22 described above inconnection with FIGS. 2 and 5. One suitable device for acceptingbiometric fingerprint or thumbprint data is the FCD4B14 FingerChip™available from AtMel Corporation, San Jose, Calif. As depicted in FIG.8( a), the print sensor 138 is disposed such that a finger 140 may bedrawn across print sensor 138 while in continuous contact therewith.Continuous contact is important and is facilitated by providing a curvedsurface 142 generally as shown. As finger 140 moves over print sensor138, a stream of digital biometric data is produced by print sensor 138and presented on a data bus (not shown) for manipulation bymicroprocessor-controlled logic (not shown). Clearly the exact biometricdata stream depends not only on the fingerprint but on the direction andmanner in which finger 140 is drawn across print sensor 138. This is animportant feature of this invention because it adds considerablesecurity to helmet-mounted response unit 22 in that the availability ofthe proper finger or thumb alone is insufficient to satisfy thebiometric security requirement; the user must also recall and repeatwith reasonable accuracy the exact manner in which the thumb or fingerwas drawn across print sensor 138 when first storing the user'sbiometric ID data in response unit 22.

FIG. 8( b) shows another mounting arrangement suitable for print sensor138, although the curved surface 142 is preferable. FIG. 8( c) shows yetanother example employing the curved surface 144, which importantlypermits continuous contact between finger 140 and print sensor 138 whilemoving thereover.

FIG. 8( d) demonstrates an exemplary arrangement for placing printsensor 138 within helmet-mounted response unit 22 so that the user maydraw the thumb 146 over print sensor 138 while grasping helmet-mountedresponse unit 22 preparatory to donning same. Any useful audio and/orvisual indicator means (not shown) may be provided to inform the userthat helmet-mounted response unit 22 has been successfully activated,thereby affording the opportunity to retry activation by repeating themovement of thumb 146 across print sensor 138. If desired, a therepeated attempts may be accumulated against a limited number followedby self-destruction of all logic and data stored in helmet-mountedresponse unit 22.

In an alternative embodiment of helmet-mounted response unit 22, thebiometric data necessary to identify several members of a combat unitmay be stored in biometric ID storage 66 (FIG. 5) so that any combatunit member may activate helmet-mounted response unit 22. This permits amember of the combat unit to “borrow” the helmet and its response unit22 (FIG. 2) from another member of the same combat unit and successfullyactivate it for use in the battlefield. The size of the group ofauthorized users is limited only by the memory available in biometric IDstorage 66, which may be loaded by mass-transfer of digital datacollected from the thumb-scans of all members of the combat unit.Similarly, in an alternative embodiment of interrogatory unit 20, thebiometric data necessary to identify several members of a combat unitmay be stored in biometric ID storage 114 (FIG. 6) thereby permitting amember of the combat unit to “borrow” the weapon and response unit 22(FIG. 2) from another member of the same combat unit and successfullyactivate it for use in the battlefield.

FIG. 9 is a block diagram showing a flow chart exemplifying the IFFtransaction method of this invention. This process begins with the step148 where the interrogating soldier triggers an interrogation command atinterrogatory unit 20 (FIGS. 1, 3 and 6). In the step 150, a TCOD iscreated by accepting the RGN produced in the step 152 and the CODretrieved from local memory in the step 154. In the step 156, an IRtransmit signal encoding the TCOD is projected to the targeted responseunit. In the step 158, the TCOD is received at the targetedhelmet-mounted response unit. More precisely, the 3-pulse frame-synchpreamble and the first six symbols of the TCOD are first received anddecoded in step 158, which may also include an arrival quadrantindication step (not shown) to notify the helmet wearer by some usefulmeans of the quadrant (Front, Right, Rear, or Left) from which thereceived TCOD has arrived. In the step 160, the received TCOD isvalidated by first verifying that the biometric security has beensatisfied in the step 162 and then retrieving from local storage the CODin the step 164. As described above in connection with FIG. 7, the CC(the first four symbols from the COD) is compared with the CC from thereceived TCOD and, if matched, the next two symbols of the received TCODare decoded as the RGN in the step 166. In the step 168, the RGN fromstep 166 and the RC (the fifth and sixth symbols of the COD) are used tocompute the closed shutter interval (the delay interval) of the RCOD andthe fourth symbol of the COD is used to compute the open shutterinterval (the response pulse stream) of the RCOD. In the step 170, theRCOD from step 168 is used to cycle the obturator, therebyretroreflecting the interrogatory pulse stream portion of the TCODaccording to the RCOD.

In the step 172, the RCOD is received and decoded at the interrogatoryunit and validated in the step 174, which uses the locally-stored RGNand COD retrieved in the step 176 to duplicate the computations used tocreate the RCOD at the response unit and to compare the received RCODwith the locally-computed RCOD. In the step 178, the results of step 174are evaluated to make a friend or foe decision, completing the IFFtransaction initiated in step 148. If Friend, then the step 180 signalsthe interrogatory unit user in some useful manner, such as lighting upan LED for a short time. If Foe, then the step 182 initiates arepetition of the IFF transaction (with a new RGN) or does nothing,thereby indicating that the IFF transaction has failed to identify afriend.

Clearly, other embodiments and modifications of this invention may occurreadily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawing.

1. A method for identifying, as friend or foe, a combat response unithaving a helmet-mounted challenge receiver and retroreflector obturator,the method comprising the unordered steps of: (a) projecting an infrared(IR) transmit signal including a transmitted code of the day (TCOD)encoded as pulse positions within a predetermined frame interval ontothe combat response unit from a combat interrogatory unit; (b) receivingthe IR transmit signal and TCOD at the challenge receiver; (c)selectively reflecting the IR transmit signal to modulate the reflectedsignal by opening and closing the retroreflector obturator according toa response code of the day (RCOD); (d) receiving the reflected IRtransmit signal and RCOD at the combat interrogatory unit; and (e)combining the received RCOD with the TCOD to identify the combatresponse unit as friend or foe.
 2. The method of claim 1 furthercomprising the steps of: (f) combining a first code of the day (COD)stored at the combat interrogatory unit with a randomly-generated number(RGN) to produce the TCOD; and (g) combining the received TCOD with asecond COD stored at the combat response unit to produce the RCOD. 3.The method of claim 2 further comprising the step of: (h) deactivatingthe combat response unit responsive to a doffing of the helmet.
 4. Themethod of claim 3 further comprising the step of: (i) acceptingbiometric data at the combat response unit; and (j) activating thecombat response unit responsive to the biometric data.
 5. The method ofclaim 4 wherein the combat interrogatory unit includes a weapon-mountedinterrogatory transceiver for projecting and receiving the IR transmitsignal.
 6. The method of claim 5 further comprising the step of: (k)generating an arrival quadrant signal representing the direction ofarrival of the IR transmit signal at the combat response unit.
 7. Themethod of claim 1 further comprising the step of: (f) deactivating thecombat response unit responsive to a doffing of the helmet.
 8. Themethod of claim 1 further comprising the step of: (f) acceptingbiometric data at the combat response unit; and (g) activating thecombat response unit responsive to the biometric data.
 9. The method ofclaim 1 wherein the combat interrogatory unit includes a weapon-mountedinterrogatory transceiver for projecting and receiving the IR transmitsignal.
 10. The method of claim 1 further comprising the step of: (f)generating an arrival quadrant signal representing the direction ofarrival of the IR transmit signal at the combat response unit.
 11. Asystem for combat identification as friend or foe (IFF) communicationscomprising: a combat interrogatory unit comprising projector means forprojecting an infrared (IR) transmit signal including a transmitted codeof the day (TCOD) encoded as pulse positions within a predeterminedframe interval, receiver means for receiving a reflected IR transmitsignal including a response code of the day (RCOD), and means forcombining the received RCOD with the TCOD to identify the source of thereflected IR transmit signal as friend or foe; and a helmet-mountedcombat response unit comprising sensor means for receiving a projectedIR transmit signal including the TCOD, retroreflector means forreflecting an incoming IR transmit signal generally back along theincoming path thereof, obturator means for obstructing theretroreflector means to prevent reflection thereby, and means foropening and closing the obturator means according to the RCOD.
 12. Thesystem of claim 11 further comprising: in the combat interrogatory unit,means for combining a first stored code of the day (COD) with arandomly-generated number (RGN) to produce the TCOD; and in thehelmet-mounted combat response unit, means for combining the receivedTCOD with a second stored COD to produce the RCOD.
 13. The system ofclaim 12 further comprising: in the helmet-mounted combat response unit,means for deactivating the combat response unit responsive to a doffingof the helmet.
 14. The system of claim 13 further comprising: in thehelmet-mounted combat response unit, means for accepting biometric dataat the combat response unit; and means for activating the combatresponse unit responsive to the biometric data.
 15. The system of claim14 further comprising: in the helmet-mounted combat response unit, meansfor generating an arrival quadrant signal representing the direction ofarrival of the IR transmit signal.
 16. The system of claim 15 furthercomprising: in the combat interrogatory unit, means for fixing theprojector means and the receiver means to a weapon.
 17. The system ofclaim 11 further comprising: in the helmet-mounted combat response unit,means for deactivating the combat response unit responsive to a doffingof the helmet.
 18. The system of claim 17 further comprising: in thehelmet-mounted combat response unit, means for accepting biometric dataat the combat response unit; and means for activating the combatresponse unit responsive to the biometric data.
 19. The system of claim18 further comprising: in the combat interrogatory unit, means forfixing the projector means and the receiver means to a weapon.
 20. Thesystem of claim 11 further comprising: in the helmet-mounted combatresponse unit, means for accepting biometric data at the combat responseunit; and means for activating the combat response unit responsive tothe biometric data.
 21. The system of claim 11 further comprising: inthe helmet-mounted combat response unit, means for generating an arrivalquadrant signal representing the direction of arrival of the IR transmitsignal.
 22. The system of claim 11 further comprising: in the combatinterrogatory unit, means for fixing the projector means and thereceiver means to a weapon.
 23. A combat response unit adapted formounting in a helmet for use in a combat identification as friend or foe(IFF) communications system, the combat response unit comprising: meansfor receiving a projected infrared (IR) transmit signal including atransmitted code of the day (TCOD), the TCOD including a periodicallyupdated code of the day (COD) portion and a randomly generated portion;retroreflector means for reflecting an incoming IR transmit signalgenerally back along the incoming path thereof; obturator means forobstructing the retroreflector means to prevent reflection thereby;means for storing a reprogrammable Code of the Day (COD); and means foropening and closing the obturator means according to a response code ofthe day (RCOD) based in part on the COD.
 24. The unit of claim 23further comprising: means for combining the received TCOD with a secondstored the COD to produce the RCOD.
 25. The unit of claim 24 furthercomprising: means for deactivating the combat response unit responsiveto a doffing of the helmet.
 26. The unit of claim 25 further comprising:means for accepting biometric data at the combat response unit; andmeans for activating the combat response unit responsive to thebiometric data.
 27. The unit of claim 23 further comprising: means fordeactivating the combat response unit responsive to a doffing of thehelmet.
 28. The unit of claim 27 further comprising: means for acceptingbiometric data at the combat response unit; and means for activating thecombat response unit responsive to the biometric data.
 29. The unit ofclaim 23 further comprising: means for accepting biometric data at thecombat response unit; and means for activating the combat response unitresponsive to the biometric data.
 30. The system of claim 23 furthercomprising: in the helmet-mounted combat response unit, means forgenerating an arrival quadrant signal representing the direction ofarrival of the IR transmit signal.